Conveyorized vacuum applicator and method of applying a dry film resist to a printed circuit board

Abstract

An improved method of and apparatus that is continuously automatically operative in an in-line system is described for applying under heat, vacuum and mechanical pressure a dry film photoresist-forming layer to printed circuit boards ( 136 ) that already have been prelaminated by the loose application thereto of the dry film resist as discrete cut sheets within the confines of the surface of the boards whereby a laminate without entrapped air bubbles and closely conforming to the raised circuit traces and irregular surface contours of the printed circuit board is obtained. Featured is a conveyorized vacuum laminator ( 18 ) comprising heated upper and lower platens ( 96,94 ) adapted to be moved into sealing engagement to form a vacuum lamination chamber ( 16 ), and disposed between the platens are two independent belt conveyor systems ( 20,22 ) that act as heat shields to prevent the printed circuit board and the loosely applied dry film from being heated up too rapidly in the vacuum chamber by the residual heat given off by the upper and lower platens before the film can be completely evacuated. This prevents premature tacking of the dry film to the board and enables all of the air entrapped between the film and the board surface to be drawn off before the application of heat and mechanical pressure, so as to ensure complete conformance of the dry film around the circuit traces and surface contours.

What is claimed is:  
     
       1. A method of vacuum laminating a dry film photoresist-forming layer onto a prelaminated printed circuit board ( 136 ) which prevents premature tacking of the dry film to the board, comprising: 
       (a) placing the prelaminated board on one section of a moving lower belt conveyor ( 20 ) for movement into a vacuum chamber region ( 16 ) of a vacuum laminator ( 18 ) for the application of heat, vacuum and mechanical pressure, which lower belt conveyor is characterized in having two distinct sections ( 50 , 52 ) upon which the board can be placed, the two sections being so positioned such that, when one section of the lower belt conveyor is moved with the board into the vacuum chamber region, the other section is moved out of said region for cooling and vice versa;  
       (b) indexing one section of an upper belt conveyor ( 22 ) spaced above said lower belt conveyor into the vacuum chamber region, which upper belt conveyor is characterized in having at least two distinct sections, at least two sections being to positioned such that, when one section of the upper belt is moved into the vacuum chamber region, at least one other section is moved out of said region for cooling and vice versa;  
       (c) sensing the positioning of the board in the vacuum chamber and stopping the movement of the lower belt conveyor;  
       (d) sealing the vacuum chamber to capture the board therein;  
       (e) vacuum laminating the prelaminated board within the vacuum chamber; and,  
       (f) when the vacuum lamination process is complete, opening the vacuum chamber and moving the lower belt until the one section carrying the laminated board is moved out of the vacuum chamber region and the board is unloaded and the other section which has been allowed to cool is moved into position to receive another prelaminated board.  
     
     
       2. A method according to  claim 1 , further including the steps of: 
       (g) placing on the cool section of the lower belt conveyor ( 20 ) another prelaminated printed circuit board ( 136 ); and,  
       (h) repeating (a) through (f).  
     
     
       3. A method according to  claim 1 , further including, before step (a), the steps of: 
       (i) conveying on an input conveyor ( 14 ) a prelaminated board ( 136 ) from preceding equipment to the lower belt conveyor ( 20 ),  
       (j) providing a barrier ( 26 ) at the exit end of the input conveyor to stop the movement of the board and to align the board before proceeding onto the lower belt conveyor;  
       (k) sensing the presence of the board at the exit end of the input conveyor and stopping the movement of the input conveyor;  
       (l) adjusting the barrier to a non-blocking position; and,  
       (m) starting the input conveyor and the lower belt conveyor to load the board into the vacuum chamber region ( 16 ) of the vacuum laminator ( 18 ).  
     
     
       4. A method according to  claim 1 , wherein the vacuum laminator ( 18 ) has a relatively stationary upper platen ( 94 ) that is adapted to be moved up into sealing engagement with said upper platen to form said vacuum chamber ( 16 ) between them, said lower belt conveyor including a sectioned belt ( 48 ) under tension having two apertures ( 62 , 64 ) therein that extend across the width of the sectioned belt and divide the sectioned belt into said two distinct sections ( 50 , 52 ), the two apertures being so positioned such that, when the board is moved into the vacuum chamber region between the upper and lower platens, one aperture is moved into said region into alignment with the lower platen, and the other aperture is moved out of said region and vice versa; 
       wherein said upper belt conveyor ( 22 ) including an endless belt ( 108 ) having said distinct sections that index into an out of the vacuum chamber region of said vacuum laminator, and wherein, the board ( 136 ) has reached the vacuum chamber, the lamination process includes the steps of:  
       (n) relieving the tension on the sectioned belt;  
       (o) lifting the lower platen ( 94 ) up through the aligned aperture ( 62 , 64 ) in the sectioned belt of the lower belt conveyor into sealing engagement with the upper platen ( 96 ) and thereby capturing within the vacuum chamber the board and at least a portion of the sectioned belt of the lower belt conveyor upon which the board is positioned and at least a portion of the sectioned belt of the upper belt conveyor;  
       (p) evacuating the vacuum chamber ( 16 ) of the vacuum laminator ( 18 ) for a time sufficient to draw off all of the air between a loosely applied prelaminated dry film and the surface of the board and to thereby place the dry film in intimate contact with the board surface;  
       (q) heating the upper platen ( 96 ) and lower platen ( 94 ) of the vacuum laminator to a temperature at which the dry film laminate on the board has a high flow characteristic;  
       (r) causing the upper platen ( 96 ) to apply mechanical pressure to the board to force the heated laminate to conform closely to the surface contours of the board;  
       (s) when the vacuum lamination process is complete, allowing atmospheric air to enter the vacuum chamber;  
       (t) lowering the lower platen ( 94 ) down through the aperture in the belt of the lower belt conveyor;  
       (u) restoring the tension in the sectioned belt ( 48 );  
       (v) conveying the board out of the vacuum laminator; and,  
       (w) continuing the movement of the sectioned belt such that the cool section thereof is moved into position for acceptance of a new prelaminated board.  
     
     
       5. A method according to  claim 4 , wherein the upper platen ( 96 ) of the vacuum laminator ( 18 ) includes a diaphragm ( 114 ) that is substantially impervious to air and forms the ceiling of the vacuum chamber and wherein: 
       (x) mechanical force is applied by:  
       (y) evacuating the space between the diaphragm and the upper platen; and,  
       (z) allowing atmospheric air or compressed air to enter the space between the diaphragm and the upper platen thereby to cause the diaphragm to slap down and apply mechanical pressure of the board to force dry film laminate to conform closely to the surface contours of the board.  
     
     
       6. A method according to  claim 1 , wherein the dry film is a solder mask or a primary imaging resist.